Add some README files to give an overview of each artifacts

mug-errorprone/README.md

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+# Compile Time Plugin for `StringFormat`
+
+The `com.google.mu.util.StringFormat` class in Mug and `com.google.mu.safesql.SafeQuery` class provide parsing and formatting functionality based on a string template. This artifact provides compile-time checks to help using these classes safely.
+
+If you use bazel, the `mug:format` and `mug-guava:safe_sql` build targets export the plugin out of the box.
+
+If you use Maven, add the following POM snippet to your `maven-compiler-plugin`:
+
+```
+<configuration>
+  <annotationProcessorPaths>
+    <path>
+      <groupId>com.google.errorprone</groupId>
+      <artifactId>error_prone_core</artifactId>
+      <version>2.23.0</version>
+    </path>
+    <path>
+      <groupId>com.google.mug</groupId>
+      <artifactId>mug-errorprone</artifactId>
+      <version>7.0</version>
+    </path>
+</configuration>
+```

mug-guava/README.md

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+# Using Mug with Guava
+
+This artifact provides utilities with both Mug and Guava dependencies.
+
+* `SafeQuery` builds SQL queries using the same template syntax as `StringFormat`, while preventing SQL injection.
+* `GuavaCollectors` provide extra `Collector` and `BiCollector`s.
+* `CaseBreaker` helps breaking apart camelCase, snake_case, UPPER_SNAKE_CASE and dash-case words.
+* `BinarySearch` is a generic binary search algorithm that can be used for use cases beyond `List` and arrays.
+* `Immutables` has more convenient factory methods for Guava immutable collections.

mug-protobuf/README.md

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+Extra protobuf utilities